Method of manufacturing p-type gallium oxide by intrinsic doping, the thin film obtained from gallium oxide and its use

ABSTRACT

The inventive method provides for a method of p-type doping of Ga 2 O 3  without adding impurity elements. Embodiments allow for significant simplification relative to extrinsic impurity element doping, and thus offers a reduced fabrication cost while being more temperature resistant since the defect dopants require higher temperatures to alter their impact. Certain methods disclosed provide for p-type gallium oxide formation via intrinsic defect doping, without requiring the addition of impurity elements which provide significant simplification relative to the existing state of the art approaches providing more temperature and radiation resistance, while offering a reduced fabrication cost.

CROSS-REFERENCE TO RELATED APPLICATION

This invention claims priority from French Application No. FR1904117,filed Apr. 17, 2019, which is incorporated by reference herein in itsentirety.

FIELD OF THE INVENTION

This invention concerns a procedure for the growth of p-type Ga₂O₃(gallium oxide). More specifically, the object of the invention is aprocess for the fabrication of p-type Ga₂O₃ (phase α, β, ε or κ) bydefect doping, without adding an impurity element, as well as the thinfilm obtained by the aforementioned process, along with the utilisationof the p-type Ga₂O₃ thin film in p-n junctions in, for example, powerelectronics, high frequency electronics, thermoelectrics, UVCphotodetectors etc.

BACKGROUND OF THE INVENTION

Gallium oxide is a sesquioxide inorganic compound (i.e. with the formulaGa₂O₃). It exists in the form of several polymorphs or phases. Five suchphases are recognised: α, β, γ, δ and ε (and its' variant κ). Amongstthese, the β phase is the most stable, both chemically and thermally,with a melting point of around 1900° C.

Gallium (III) oxide has been studied for use in lasers, light emittersand luminescent materials. Ga₂O₃ is used in gas sensors as a lightemitter and can be used as a dielectric layer in solar cells. Thisthermodynamically-stable oxide also shows potential for use as a deep UVtransparent conductor and as channel layers in power electronictransistors.

Gallium oxide can be synthesised by numerous methods, the specificfabrication conditions of which determine the film properties. Thephysical properties of the elaborated thin films are governed by thegeneral growth parameters, the nature and orientation of the substrate,and also be the choice of the growth method. Gallium oxide, β-Ga₂O₃, isan intrinsically dielectric material. It can also show, under certaingrowth conditions, n-type semiconductor properties, with resistivityvalues as low as 10⁻² Ω·cm, which can be explained by the existence ofoxygen vacancies or by an extrinsic doping (with an impurity elementsuch as silicon or tin).

The conductivity of β-Ga₂O₃ crystals can be controlled by modifying thegaseous environment or by impurity doping. The conductivity can beincreased by reducing the oxygen content. It is suggested that thepresence of hydrogen also plays a role in the electrical conductivityobserved in doped β-Ga₂O₃. Furthermore, doping with other elements alsoaffects the electrical conductivity and the density of free electrons inβ-Ga₂O₃ crystals. For example, elements of group IV such as Si, Ge andSn substitute for Ga, or group VII elements such as Cl or F, cansubstitute for O and act as shallow donors.

The patent filing U.S. 20080038906 describes a method to produce ap-type thin film of Ga₂O₃ and a thin film Ga₂O₃ p-n junction. The methodinvolves a first stage of producing a thin film, of Ga₂O₃ with reducedoxygen vacancy density and a second stage of forming a p-type Ga₂O₃film, by doping the Ga₂O₃ with an acceptor. The acceptor used in thismethod is magnesium, Mg.

The patent filing U.S. 20120304918 describes a growth technique forGa₂O₃, an electroluminescent device based on Ga₂O₃ and its' fabricationmethod. More specifically, it reveals a growth procedure for a p-typeGa₂O₃ film which replaces the Ga in the film with an acceptor dopantchosen from H, Li, Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Ba, Ra, Mn, Fe,Co, Ni, Pd, Cu, Ag, Au, Zn, Cd, Hg, Tl and Pb.

We know from the state-of-the-art that gallium oxide (α, β, ε or κ) isnatively n-type. We also know that n-type doping can be enhanced byadding impurity elements which act as shallow donors (typical examplesare germanium, tin, fluorine, chlorine or silicon). We notice in theabove documents that β-Ga₂O₃ can be p-type doped by extrinsic dopingduring growth via introduction of impurities, which can act as shallowacceptors. The addition of the impurity requires controlled andreproducible introduction of a third element in the Ga₂O₃, whichconstitutes a significant complication of the manufacturing process.Amongst other issues, the dopant can be mobile in the Ga₂O₃ crystalstructure and hence modify the electrical properties over time. Suchdopant diffusion can also be provoked by elevated temperatures, such ascan be expected in high temperature electronics, thermoelectrics powerelectronics, space electronics and other applications, including someunexpected situations which can challenge the thermal robustness.

SUMMARY OF INVENTION

This invention proposes a method to do p-type doping of Ga₂O₃ withoutadding impurity elements. This intrinsic doping approach represents asignificant simplification relative to extrinsic impurity elementdoping, and thus offers a reduced fabrication cost. Moreover, the p-typedoping of Ga₂O₃ obtained in this way is more temperature resistant sincethe defect dopants require higher temperatures to alter their impact.

More specifically, the object of the invention is a process for thefabrication of wide bandgap p-type Ga₂O₃ (phase α, β, ε or κ) by defectdoping, involving the growth of gallium oxide on a substrate (using aconventional thin film growth technique) by acting only on the oxygenstoichiometry, through control of, for instance, growth temperature, thenature/pressure of the oxidant gas, the power of an oxygen plasma cell(creating oxygen radical atoms) or the growth rate, etc. such thatp-type gallium oxide is formed via intrinsic defect doping, withoutrequiring the addition of impurity elements.

According to a variation of the invention, the substrate can be sapphire(Al₂O₃) (with a, c, r or m orientation), strontium titanate (SrTiO₃);magnesium oxide (MgO), nickel oxide (NiO), copper oxide (CuO or Cu₂O),silicon (Si), gallium oxide (Ga₂O₃), silicon carbide (SiC-4H or SiC-6H),zinc oxide (ZnO), gallium nitride (GaN), aluminium nitride (AlN), glass,quartz, a metal, graphene, graphite, graphene oxide, a polymer, etc.

According to a variation of the invention, the growth of gallium oxideon a substrate can be done by liquid phase deposition, sol-gel, physicalvapor deposition (PVD), thermal evaporation, electron-beam evaporation,DC magnetron sputtering, pulsed laser deposition (PLD), molecular beamepitaxy (MBE), chemical vapor deposition (CVD), metal organic CVD,atomic layer deposition (ALD), etc.

More specifically, the growth temperature can be either held constant orvaried (over a temperature range between 300 and 1500° C.) and thepartial pressure can also be held constant or varied (according to thelevel/profile of doping that is desired) between 10⁻² and 10⁻¹¹ Torr.

The oxidant atmosphere can be gaseous molecular oxygen, water vapor,gaseous ozone, gaseous nitrogen monoxide or dioxide (NO or NO₂) oratomic oxygen from a plasma source.

In a certain variant the procedure can involve the use of a buffer layer(chosen for materials parameters such as the crystal structure, thelattice parameters, the crystalline orientation, the layer thickness,the surface morphology and/or the desired grain size), which can be inan oxide or nitride material. The thickness of the buffer layer can varyfrom 1 to 500 nm.

In a certain variant the procedure can involve a post-anneal (in-situ orex-situ) in order to reinforce the activation of the p-type doping inthe gallium oxide. This anneal can last between 10 ns and 10 hours.

In a certain variant, the anneal can be thermal or by laser or by lamp.The anneal can be at a fixed temperature or at a variable temperature.The anneal temperature is, between 600 and 1500° C. The atmosphere forthe anneal can be molecular oxygen, NO₂, NO, ozone, N₂, Ar, Kr or air.

In a certain variant, the laser anneal can be realised with UV lighthaving a wavelength under that corresponding to the bandgap of Ga₂O₃(i.e. an energy≥5 eV). The length of exposure can be between 10 ns and 1s. The energy density of the light source can be between 1 mJ/cm² and 1kJ/cm².

In a certain variant of the invention, the lamp anneal can be done witha halogen lamp or an incandescent lamp or a discharge lamp, with anannealing temperature which can be up to 1500° C. The energy density canbe over 100 J/cm² and last between 100 μs and 300 seconds.

The invention concerns a p-type Ga₂O₃ thin film (α, β, ε or κ) with athickness between 1 nm and 10 microns by defect doping, characterised inthat it is realised through the growth of gallium oxide on a substrate,by intrinsic p-type defect doping without intentionally adding animpurity element according to the procedure described above.

The invention also concerns the use of p-type Ga₂O₃ thin films (α, β, εor κ), obtained by the above procedure, in Ga₂O₃ p-n junctions.

Moreover, the invention concerns the use of p-type Ga₂O₃ thin films (α,β, ε or κ), obtained by the above procedure as p-type transparentelectrodes or as UVC photodetectors or as high frequency switches.

The invention also concerns, the utilisation of the p-type Ga₂O₃ thinfilm, obtained by the above process, in high temperature electronics,power electronics, high frequency electronics, thermoelectrics, or spaceelectronics.

DETAILED DESCRIPTION OF THE INVENTION

In order to exploit the growth process for p-type gallium oxide, one canuse, for example, a thin film deposition tool such as molecular beamepitaxy, DC magnetron sputtering, laser ablation (sometimes calledpulsed laser deposition (PLD)) or chemical vapor deposition (CVD) ormetal organic CVD or liquid phase deposition or sol gel or atomic layerdeposition.

The substrate can be sapphire (Al₂O₃), strontium titanate (SrTiO₃);magnesium oxide (MgO), nickel oxide (NiO), copper oxide (CuO or Cu₂O),silicon (Si), gallium oxide (Ga₂O₃), silicon carbide (SiC-4H or SiC-6H),zinc oxide (ZnO), gallium nitride (GaN), aluminium nitride (AlN), glass,quartz, a metal, graphene, graphite, graphene oxide, a polymer, etc. Thesubstrate is cleaned and brought to a sufficiently high temperature inorder to degas. The oxidant atmosphere can be gaseous molecular oxygen,water vapor, gaseous ozone, gaseous nitrogen monoxide or dioxide (NO orNO₂) or atomic oxygen from a plasma source. The background pressure canbe held constant or varied (according to the level/profile of dopingthat is desired) between 10⁻² and 10⁻¹¹ Torr.

The growth temperature can be either held constant or varied over atemperature range between 300 and 1500° C. according to the desiredlevel of doping (higher temperature reduces the doping level). The filmthickness can be between 1 nm and 10 microns. A typical growth rate isabout 0.5 nm per minute.

A buffer layer can be used in order to increase the carrierconcentration and/or mobility in the p-type Ga₂O₃ layer, or in order toincrease the conductivity of the p-Ga₂O₃. The thickness of the bufferlayer can be between 1 nm and 500 nm. In order to boost the activationof the p-type doping, a post-anneal step can be used. The anneal can bethermal or by laser or by lamp. In the case of a thermal anneal theanneal can be at a fixed or variable temperature between 600 and 1500°C. The thermal anneal can last between 10 minutes and 10 hours. Theatmosphere for the anneal can be molecular oxygen, NO₂, NO, ozone, N₂,Ar, Kr or air.

The laser anneal (continuous wave or pulsed) can be realised with UVlight having a wavelength under that corresponding to the bandgap ofGa₂O₃ (i.e. an energy≥5 eV). The length of exposure can be between 10 nsand 1 s. The energy density of the light source can be between 1 mJ/cm²and 1 kJ/cm².

In the case of a lamp anneal the source is often a halogen lamp. Thistechnology is relatively flexible in terms of temperature rise and thusallows large thermal dynamic of the anneal. The annealing temperaturecan be up to 1500° C. The energy density can be over 100 J/cm² and lastbetween 100 μs and 30 seconds.

The growth of a thin film of p-type Ga₂O₃ in the currentstate-of-the-art is achieved by introducing a p dopant such that the Gain the Ga₂O₃ layer is replaced by the said dopant. For the currentinvention there is no introduction of a dopant; the procedure is basedon varying only the oxygen stoichiometry, through control of growthconditions, for instance, growth temperature and/or the nature/pressureof the oxidant gas and/or the growth rate etc. With the procedure of theinvention, p-type gallium oxide is formed via intrinsic defect doping,without requiring the addition of impurity elements. This approachrepresents a significant simplification relative to the existing stateof the art approaches, and thus offers a reduced fabrication cost.Moreover, the p-type doping of Ga₂O₃ obtained in this way is moretemperature and radiation resistant since the defect dopantdistributions are not altered as readily as impurity defectdistributions at elevated temperature.

We will now give a descriptive example of a procedure for thefabrication of p-type Ga₂O₃ according to the present invention. In this,the p-type Ga₂O₃ is formed on a c-plane oriented sapphire (c-Al₂O₃)substrate by pulsed laser deposition (often referred to as PLD or laserablation). The substrate is cleaned by sequential immersion in acetone,ethanol and deionised water followed by drying in a flow of drynitrogen. It is then heated, under vacuum (<10⁻⁶ Torr), to a temperaturebetween 700+/150° C. and held there for 30 minutes in order to degas.The growth is conducted with a Coherent LPX 200 KrF (248 nm) or ArF (193nm) excimer laser (at a pulse repetition rate of 10+/−8 Hz and a pulseduration of 30+/−15 ns) and a solid source comprised of acompressed/sintered stoichiometric 4N powder of Ga₂O₃. The beam isfocused on the target in order to give a power density of about10⁸+/−5×10⁷ W/cm².

A uniform coverage of the two-inch-diameter c-sapphire wafer is obtainedusing an optical rastering of the incident laser beam on the target. Thetemperature of the Al₂O₃ target is maintained between 380 and 550° C.and the oxygen pressure during the growth is 10⁻⁴ Torr. In this way weobtain films of ε/κ-Ga₂O₃ of about 300 nm thick. The growth rate isabout 5 nm/min. The films are then air annealed in a tubular furnace at700+/−100° C. for 30 minutes in order to activate the concentration ofp-type carriers. The Al₂O₃ substrate, on which the Ga₂O₃ is grown, hasthe advantage of being cheaper than the Ga₂O₃ bulk substrate that istypically used in the state-of-the-art, and it can be found in bothlarger formats and much larger production volumes. This makes thecurrent invention's approach to p-type Ga₂O₃ fabrication cheaper andmore suited to a rapid industrialisation.

The invention also concerns p-type Ga₂O₃ (α, β, ε or κ) thin filmsobtained by the above process. The films obtained can have thicknessesbetween 1 nm and 10 micrometers according to the growth time.

The invention also concerns the use of films of p-type Ga₂O₃ (α, β, ε orκ) obtained by the above process in p-n junctions, in which the n-typeand p-type regions are both made from intrinsically-doped Ga₂O₃according to the above process.

Moreover, the invention concerns the use of p-type Ga₂O₃ thin films (α,β, ε or κ), obtained by the above procedure in UVC photodetectors,p-type transparent electrodes, radiation resistant electronics (e.g.betavoltaics), high temperature electronics, thermoelectrics, powerelectronics, high frequency electronics, space electronics and otherapplication domains. For example, the performance of a field effecttransistor (FET), realised in a thin film of beta gallium oxide oninsulator, were presented in the article “High PerformanceDepletion/Enhancement-Mode β-Ga₂O₃ on Insulator (GOOI) Field EffectTransistors” published in IEEE Electron Device Letters in January 2017.

We know that the ultrawide bandgap of p-type Ga₂O₃ (α, β, ε or κ) makesit relatively efficient for use in high voltage/frequency switches andthat this enhanced efficiency could help to reduce the energyconsumption by replacing the silicon based devices which are currentlyused in high power/frequency switches.

The study and structural/chemical analysis of a film of p-Ga₂O₃ obtainedaccording to the current invention, as well as the electricalproperties, were published in the article “p-type β-Ga₂O₃ oxide; A newperspective for power and optoelectronic devices” in Materials TodayPhysics.

The invention claimed is:
 1. A method for the fabrication of widebandgap p-type Ga₂O₃ (α, β, ε or κ) by defect doping with galliumvacancies, involving the growth of gallium oxide on a substrate, using aconventional thin film growth technique, by acting only on the oxygenstoichiometry, and through controlling the growth temperature, thenature/pressure of the oxidant gas or the growth rate, such that p-typegallium oxide is formed via intrinsic defect doping, without requiringthe addition of impurity elements.
 2. The method for the fabrication ofp-type Ga₂O₃ (α, β, ε or κ) according to claim 1 in which the substratecan be sapphire (Al₂O₃) (with a, c, r or m orientation), strontiumtitanate (SrTiO₃); magnesium oxide (MgO), silicon (Si), gallium oxide(Ga₂O₃), silicon carbide (SiC-4H or SiC-6H), nickel oxide (NiO), copperoxide (CuO or Cu₂O), zinc oxide (ZnO), gallium nitride (GaN), aluminiumnitride (AlN), glass, quartz, a metal, graphene, graphite, grapheneoxide, or a polymer.
 3. The method for the fabrication of p-type Ga₂O₃(α, β, ε or κ) according to claim 1 in which the growth temperature canbe either held constant or varied over a temperature range between 300and 1500° C. and in which the partial pressure of oxidant gas can beheld constant or varied (according to the level/profile of doping thatis desired) between 10⁻² and 10⁻¹¹ Torr.
 4. The method for thefabrication of p-type Ga₂O₃ (α, β, ε or κ) according to claim 1 in whichthe oxidant atmosphere can be gaseous molecular oxygen, water vapor,gaseous ozone, gaseous nitrogen monoxide or dioxide (NO or NO₂) oratomic oxygen from a plasma source.
 5. The method for the fabrication ofp-type Ga₂O₃ (α, β, ε or κ) according to claim 1 which involves the useof a buffer layer, which can be in an oxide or nitride material, whereinthe thickness of the buffer layer is between 1 to 500 nm.
 6. The methodfor the fabrication of p-type Ga₂O₃ (α, β, ε or κ) according to claim 1which involves a post-anneal (in-situ or ex-situ) in order to reinforcethe activation of the p-type doping in the gallium oxide, wherein theanneal occurs over a duration between 10 ns and 10 hours.
 7. The methodfor the fabrication of p-type Ga₂O₃ (α, β, ε or κ) according to claim 6which involves an anneal.
 8. The method for the fabrication of p-typeGa₂O₃ (α, β, ε or κ) according to claim 6 in which the laser anneal canbe realised with UV light (continuous or pulsed) having a wavelengthunder that corresponding to the bandgap of Ga₂O₃ (an energy of ≥5 eV).9. The method for the fabrication of p-type Ga₂O₃ (α, β, ε or κ)according to claim 6 in which the lamp anneal can be done with a halogenlamp or an incandescent lamp or a discharge lamp, with an annealingtemperature which can be up to 1500° C., and, in which, the energydensity can be over 100 J/cm² and last between 100 μs and 300 seconds.10. A thin film of p-type Ga₂O₃ (α, β, ε or κ) with a thickness between1 nm and 10 μm that is obtained by the growth of gallium oxide on asubstrate via intrinsic defect doping, without requiring the addition ofimpurity elements according to the fabrication method of claim
 1. 11.Use of a p-type Ga₂O₃ (α, β, ε or κ) thin film, obtained by thefabrication method of claim 1, in Ga₂O₃ based p-n junctions.
 12. Use ofa p-type Ga₂O₃ (α, β, ε or κ) thin film, obtained by the fabricationmethod of claim 1, as a p-type transparent electrode, in UVCphotodetectors, in high frequency switches, in high temperatureelectronics, in power electronics, in thermoelectrics, in radiationresistant electronics (e.g. betavoltaics) and in space electronics. 13.The method-for the fabrication of p-type Ga₂O₃ (α, β, ε or κ) accordingto claim 7 wherein the anneal is a thermal anneal, a laser anneal, alamp anneal, or combinations thereof.
 14. The method-for the fabricationof p-type Ga₂O₃ (α, β, ε or κ) according to claim 7 wherein the annealis at a fixed temperature or at a variable temperature.
 15. Themethod-for the fabrication of p-type Ga₂O₃ (α, β, ε or κ) according toclaim 7 wherein the atmosphere for the anneal can be molecular oxygen,NO₂, NO, ozone, N₂, Ar, Kr or air.
 16. The method-for the fabrication ofp-type Ga₂O₃ (α, β, ε or κ) according to claim 8 wherein the length ofexposure is between 10 ns and 1 s, and wherein the energy density of thelight source can be between 1 mJ/cm² and 1 kJ/cm².